1. Field of the Invention
The present invention relates to a glass-fiber-reinforced thermoplastic resin composite stampable material which can be shaped by stamping and which excels in strength and modulus of elasticity, as well as in moldability, and, in addition, exhibits a high degree of uniformity of color. The invention also is concerned with a method of producing such a stampable material.
2. Description of the Related Arts
Techniques have been known for fabricating composite materials in which fibers having a high modulus of elasticity, such as glass fibers, are added to a matrix material such as a thermoplastic resin, so as to obtain high degrees of strength and rigidity in addition to the formability inherently possessed by the thermoplastic resin. This type of composite material, referred to as glass-fiber-reinforced thermoplastic composite material, is used in structural members which are required to have high degrees of rigidity and strength and reduced weight. Among known composite materials of the kind described, plate or sheet-type composite materials, which contain comparatively long fibers, are attracting attention for use as materials of large-sized structural members. Such materials are usually shaped after being heated to a temperature above the softening point of the matrix resin. A process analogous to a web-making machine of the type used in the papermaking industry has been noticed as a representative method for forming plate or sheet-type composite materials suitable for production of large-sized structural members by stamping using a press.
In general, the web-making process includes the steps of dispersing long glass fibers and particles of thermoplastic resin in an aqueous solution of a surfactant containing fine bubbles of gas, holding the dispersion liquid on a porous supporting plate so as to form a sheet-like web in a manner similar to preparation of paper sheet, and applying heat and pressure to the web to prepare a solidified and dense sheet-like glass-fiber-reinforced thermoplastic composite material. This technique is disclosed, for example, in Japanese Patent Publication No. 2-48243 and Japanese Patent Laid-Open No. 60-158227.
The present invention pertains to a stamping glass-fiber-reinforced thermoplastic resin composite material produced by a web-making-type process. Excellent mechanical properties such as strength and modulus of elasticity of the stampable material owe to the fact that the load applied to the resin matrix which inherently has low levels of strength and modulus of elasticity is sufficiently borne by the glass fibers which possess high levels of strength and modulus of elasticity. The sharing of the load by the glass fibers is realized through the interface between the fibers and the resin, i.e., through the exterior surfaces of the fibers. Therefore, mechanical properties of the stampable material and structural members formed from this material are improved when the fibers exhibit greater wettability to the matrix resin and when the bonding strength between the fibers and the matrix resin is high.
Unfortunately, however, mechanical properties of known stampable materials produced by web-making-type processes are still unsatisfactory.
As a measure for improving wettability and bonding strength, a method has been disclosed in Japanese Patent Laid-Open No. 63-41128 in which a silane coupling agent is added to the web. However, it has been recognized that mere addition of a silane coupling agent cannot provide sufficiently high wettability and bonding strength between the glass fibers and the matrix resin.
It might be possible to lower the melt-viscosity of the thermoplastic resin, i.e., to use a resin having a smaller molecular weight, in order to enhance the wettability and the interface as a measure for improving mechanical properties. Such a measure, however, undesirably impairs mechanical properties due to, for example, embrittlement of the matrix resin, although it improves wettability in some cases.
Improvement of bonding strength at the interface between glass fibers and matrix resin has also been discussed in the field of glass-fiber-reinforced thermoplastic resin composite material intended for use in an entirely different technology, namely as an injection molding material. In this field of technology, a method has been known in which, when the thermoplastic resin is a non-polar resin such as polypropylene, the polypropylene is denatured by maleic acid or maleic anhydride. For example, Japanese Patent Laid-Open No. 48-68640, as well as Japanese Patent Publication No. 51-10265, discloses a method in which denatured polypropylene and glass fibers are melted and uniformly mixed by kneading in an extruder such as a screw extruder so as to improve bonding between the glass fibers and the denatured polypropylene at their interface, thereby producing a composite material which excels both in strength and elasticity, This known technique, however, cannot be used in the production of stampable material by a web-making process or the like, because such a process does not include any way of uniformly mixing the glass fibers and the thermoplastic resin, through melting and kneading or otherwise. A mere addition of an additive such as denatured polypropylene cannot and would not provide any appreciable improvement of strength or, alternatively, causes a large fluctuation of strength, because the bonding strength and wettability at the interface between the glass fibers and the thermoplastic resin cannot be significantly improved due to the fact that there is no kneading step.
Another problem encountered in the production of stampable material through a web-making process resides in that, when the additive has been colored, unevenness of coloring appears on the surface of the sheet due to inferior dispersion of such additive. A more uniform dispersion of denatured polypropylene might theoretically be attainable by refining the denatured polypropylene before the latter is added. Refining of the denatured polypropylene, however, causes other problems such as dropping of fine particles of denatured polypropylene through the pores of the porous support plate or clogging of the pores with the fine particles.
The aforementioned known technique disclosed in, for example, Japanese Patent Laid Open No. 48-68640 and Japanese Patent Publication No. 51-10265, in which glass fibers are uniformly dispersed in a resin by melting and kneading, also suffers from still another disadvantage. When an additive such as denatured polypropylene is added, reduction of the mechanical properties of the matrix resin due to brittleness of the additive itself becomes noticeable as the content of the additive increases, all at the cost of enhancement of bonding at the interface which might serve to improve the mechanical properties of the composite material. Thus, there is a limit to the permissible amount of addition of the additive, and this limit is an impediment to significant improvement of mechanical properties of the composite material.
In the production of a stamping material using papermaking process, an attempt has been made to use, as the thermoplastic resin particles for forming the matrix resin, particles which have been denatured in advance. In such an attempt, however, deterioration of the mechanical properties due to brittleness of the denatured thermoplastic resin itself becomes noticeable at the cost of improvement in the mechanical properties which is offered by enhanced bonding at the interface after preparation of the composite material. This attempt, therefore, also fails to provide any remarkable increase in the mechanical properties of the stamping material.